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Thyroid hormones play important roles in maintaining energy homeostasis and regulating energy expenditure. Their physiologic effects, mediated at multiple target organs, are primarily to stimulate cell metabolism and activity. The vital roles of these hormones, particularly in development, differentiation, and maturation, are underscored by the severe mental retardation observed in infants with deficient thyroid hormone function during gestation. Thyroid hormones are derived from the amino acid tyrosine and are produced by the thyroid gland in response to stimulation by thyroid-stimulating hormone (TSH) produced by the anterior pituitary. TSH, in turn, is regulated by the hypophysiotropic peptide thyrotropin-releasing hormone (TRH) (Figure 4–1). Thyroid hormone production is also under regulation by dietary iodine.

Figure 4–1.

The hypothalamic-pituitary-thyroid axis. Thyrotropin-releasing hormone (TRH) is synthesized in parvicellular neurons of the paraventricular nucleus of the hypothalamus and released from nerve terminals in the median eminence, from where it is transported via the portal capillary plexus to the anterior pituitary. TRH binds to a G protein–coupled receptor in the anterior pituitary, leading to an increase in intracellular Ca2+ concentration, which in turn results in stimulation of exocytosis and release of thyroid-stimulating hormone (TSH) into the systemic circulation. TSH stimulates the thyroid gland to increase the synthesis and secretion of tetraiodothyronine (T4) and triiodothyronine (T3) into the circulation. T4 and T3 inhibit the secretion of thyrotropin both directly and indirectly by inhibiting the secretion of TRH. Additional factors that inhibit TSH release are glucocorticoids, somatostatin, and dopamine.

The thyroid gland is a highly vascular, ductless alveolar (acinar) gland located in the anterior neck in front of the trachea. The gland weighs 10–25 g and consists of a right and left lobe connected by the isthmus. The cellular composition of the thyroid gland is diverse, including the following: